The present invention relates generally to systems and methods for ultrasound and relates, more particularly, to systems and methods for ultrasound vibrometry, in which ultrasound is used to measure mechanical properties of a material or tissue of interest.
Characterization of tissue mechanical properties, particularly the elasticity or tactile hardness of tissue, has important medical applications because these properties are closely linked to tissue state with respect to pathology. For example, breast cancers are often first detected by the palpation of lesions with abnormal hardness. In another example, a measurement of liver stiffness has been used as a non-invasive alternative for liver fibrosis staging.
Recently, an ultrasound technique for measuring mechanical properties of tissues, such as stiffness and viscosity, called shear-wave dispersion ultrasound vibrometry (“SDUV”) was developed. This SDUV technique is described, for example, in co-pending U.S. Pat. Nos. 7,785,259 and 7,753,847, which are herein incorporated by reference in their entirety. In these and similar methods, a focused ultrasound beam, operating within FDA safety limits, is applied to a subject to generate harmonic shear waves in a tissue of interest. The propagation speed of the induced shear wave is frequency dependent, or “dispersive,” and relates to the mechanical properties of the tissue of interest. Shear wave speeds at a number of frequencies are measured by pulse echo ultrasound and subsequently fit with a theoretical dispersion model to inversely solve for tissue elasticity and viscosity. These shear wave speeds are estimated from the phase of tissue vibration that is detected between two or more points with known distance along the shear wave propagation path.
The shear wave speed measured with ultrasound vibrometry and related techniques is often biased such that it is greater than the true shear wave speed. This bias is position dependent and influenced by the three-dimensional structure of the ultrasound beam used to produce the shear waves. For example, the bias is larger closer to the sources of the ultrasound energy that produced the shear waves, and smaller when farther away from the sources.
In addition to the three-dimensional shape of the ultrasound push beam, the ultrasound detection beam used for shear wave detection also has a three-dimensional distribution. This means that pulse-echo detection cannot measure tissue motion at an infinitesimal point, but rather measures the averaged tissue motion within the small, but finite, detection beam dimension. This three-dimensional structure of the ultrasound detection beam can also have an impact on shear wave speed estimation. The overall result is that shear wave speed measurements are influenced by the beam shape of the ultrasound used for shear wave generation, as well as that used for detection. The ultrasound beam shape depends on where the ultrasound energy is electronically focused; therefore, shear wave speed measurements will be position dependent, even in a media with uniform stiffness, and, thus, a uniform shear wave speed.
Generally, shear wave speed measurements are depth dependent and biased towards overestimation. Shear wave speed measurements can also depend on the distance between the push beam and the detection beam. In general, measured shear wave speed is higher when detection is closer to the push beam and, thus, overestimated. This overestimation is exacerbated at shallow focal depths where the force field has split peaks.
It would therefore be desirable to provide a system and method for correcting measurements of shear wave speed for biases introduced by ultrasound push and detection beam shape and spacing.